i went to a quilting/craft store and bought a quarter-circle template and use that.
alternately, just use a string. find the center point of the circle, tie the pen to one end, hold down the other and round you go! it's a bit sloppy, but it's more accurate than freehanding.
I generally find where the circle intersects with points on the map, mark them, and then draw it one quarter circle at a time. It's easier to freehand a curved line (even a long one) than a full circle.